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The correlation between grain boundary type characterized by the coincidence site lattice(CSL) model and precipitation behavior of grain boundary chromium carbides in the thermally treated Alloy 690 (Alloy 690TT) was investigated through transmission electron microscopy. The chromium carbides precipitated in Alloy 690TT were identified as chromium-rich M_(23)C_6. They were not observed on the coherent twin boundaries (Σ3_c), and their precipitation was suppressed on nearly exact CSL boundaries such as Σ11 and Σ15. Small and faceted ones were found at the low angle boundaries with less than 15 degrees of misorientation angle. On the other hand, coarse and discrete carbides were precipitated along the random high angle grain boundaries, and needle-like ones grown parallel to the coherent twin boundary evolved on the incoherent twin boundaries (Σ3i). The grain boundary dislocations on some special boundaries were also examined to elucidate the small deviations from the exact lattice site coincidence.

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Utilizing electrical resistivity measurements during heating, precipitation behaviors of high Mg contented Al-Mg alloys, which is currently spotlighted as automotive sheet material, has been analyzed. In the attempt to analyze precipitation behavior in detail, electrical resistivity change ratio was differentiated by temperature and acquired DVRC (Differentiation values of electrical resistivity changes). By measuring DVRC during heating, more precise analysis was possible than conventional electrical resistivity measurement. The transformation process of precursor phase, such as G. P. zone, shown as positive peaks at low temperature region in DVRC, was accelerated by higher solid solution treatment temperature. This reaction was identified as being activated by excessive vacancy concentration. With increased heating rate and Mg addition, splitting of DVRC peaks in low and high temperature was observed. This indicates that in high Mg concentrated Al-Mg alloy various precipitation processes highly depend on heating rate and Mg concentration.

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In this study controlled rolling process was simulated to produce as hot rolled dual phase steel by using thermo-mechanical simulator(Gleeble 1500). Final microstructure and mechanical properties depending on finishing rolling temperature prior to intercritical heat treatment were investigated. In addition, austenite recrystallization behavior and γ/α transformation temperature of Nb containing steel were measured by various methods. The major results were as follows : the strength and elongation were improved as the finishing temperature decreased as low as 800℃ due to ferrite grain refinement by the increase of ferrite nucleation sites and acceleration of the γ/α transformation. When finishing rolling was performed at temperature below 800℃, however, the strength was decreased because of the increase of ferrite volume fraction. V and Nb were effective for retardation of austenite recrystallization so that nonrecrystallization region was extended. It was effective for ferrite refinement. Furthermore, it makes possible to perform finishing rolling at high temperature. During the intercritical heat treatment the coarsening of ferrite occurred. Repeating deformation just above Ar₃ temperature was effective for the suppression of ferrite coarsening.

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The aim of this study is to demonstrate the validity of the fracture toughness of Zr-2.5Nb pressure tube obtained from the CCT (Curved Compact Tension) specimen with a non-uniformed pre-fatigue crack. This study covers the comparison of the J-R values of the CCT specimen taken from the unirradiated Zr-2.5Nb tubes with low and high fracture toughness and the irradiated Zr-2.5Nb tube with low fracture toughness, respectively. The fracture toughness tests were carried out at RT, 250 and 300℃. At high temperatures such as 250 or 300℃, it was found that the CCT specimen with the non-uniformed pre-fatigue crack has fracture toughness, dJ/da comparable to that of the CCT specimen with the uniform pre-fatigue irrespective of fracture toughness of Zr-2.5Nb pressure tube. In contrast, at RT, the CCT specimen with the non-uniformed pre-fatigue crack had different fracture toughness from that of the CCT specimen with the uniform pre-fatigue crack, which becomes non-conservative. Especially, the CCT specimens with the non-uniform pre-fatigue crack taken from both unirradiated and irradiated Zr-2.5Nb pressure tube of low fracture toughness yield lower fracture toughness than that of the CCT specimen with the uniform pre-fatigue crack, leading to a conservatism. Finally, using both yield strength correction constant, C₁, and fracture toughness growth ratio correction constant, C₂, it is possible to predict the fracture toughness of CCT specimen with the uniform pre-fatigue crack from the result of non-uniform pre-fatigue crack.

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Ti-4Fe-(0.5, 1)Si had been investigated as a preliminary study for the development of a new titanium alloy having a low cost and high strength. The tensile strength of these alloys was comparable to that of a Ti-6Al-4V at room temperature, while lower at 400℃. For this reason, Ti-4Fe-(0.5, 1)Si-xAl (x = 2∼6 wt%) alloy system was redesigned to improve tensile strength even at 400℃ as well as at room temperature. In general, the new alloys exhibited the prior β grain size and the aspect ratio of a phase decreased with increasing Al content. The Ti_5Si₃ phase was precipitated mainly at β grain boundary in as-cast specimens. However, almost of the Ti_5Si₃ phase was transformed into Ti₃Si phase after HIP. The addition of Al to Ti-4Fe-(0.5, 1)Si alloy made the size of silicides finer, while in the case of Ti-4Fe-1Si-(2∼6)Al system coarse silicides were formed. The coarse silicides were formed mainly at grain boundary and decreased the ductility of the alloys at room temperature and at 400℃. After systematic investigation of these alloy system, suggested optimum composition is Ti-6Al-4Fe-0.5Si. Ultimate strength of this alloy was 1191 ㎫, and 930 ㎫ at room temperature, and at 400℃, respectively. These properties are superior to those of Timetal 62S as well as those of Ti-6Al-4V.

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The evolution of textures and microstructure during the warm-rolling and subsequent annealing in aluminum 3004 alloy sheets was investigated by employing X-ray texture measurements and microstructure observations. Whereas the typical β-fiber orientations with the strong Bs-orientation {112}$lt;110$gt; formed in the normally cold-rolled specimen, the warm-rolling at 250℃ led to the development of a strong through thickness texture gradient which was characterized by shear textures at the surface layer and rolling textures at the center layer. After warm rolling, ultra-fine grains formed in the thickness layer with shear texture components. Upon recrystallization annealing, the {001}$lt;100$gt; Cube-texture developed at the expense of normal rolling texture components, which gave rise to the formation of coarse recrystallized grains. However, in the layer with shear texture components the continuous recrystallization took place and the fine grain size persisted even after recrystallization annealing.

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The continuous indentation test was used to evaluate Lu¨ders strain of various steels such as ultra-fine grained steels and AISI 1020 steels. Inhomogeneous pile-up deformation in the indentation test could be related to Lu¨ders strain in tensile test. The pile-up deformation could be characterized as a localized flow phenomena. We analyzed the pile-up area as the parameter of Lu¨ders strain, and described the pile-up amount more quantitatively by considering the relation between plastic-zone size and pile-up radius. Finally, we derived the master curve for some steels, which included the ratio of elastic modulus to hardness as plastic-zone size. For various steels in this study, the values of Lu¨ders strain obtained from the master curve are consistent with those measured from the tensile test.

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Small punch test is used to assess the reliability of industrial facilities such as fusion reactor structures and power generation systems. Conventional small punch tests evaluate transition temperature and fracture strain by analyzing the load-deflection curves and deformation behaviors, respectively. However, previous research did not consider fracture mechanical concepts such as flaw, stress analysis and fracture toughness. In this study, to obtain more reliable fracture characteristics based on linear elastic fracture mechanics in small punch tests, a pre-crack was introduced to small punch specimen. Stress and deformation behaviors near the crack tip were analyzed and the stress intensity factor was derived. Using the load at crack initiation point, the fracture toughness of SA 508 Cl.3 steel was successfully evaluated.

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The effects of S and Ir on the microstructures of binary Ag-25%Pd alloy and a commercial alloy, A-2 (Ag-25 wt%Pd-2 wt%Au-2 wt%In-0.84 wt%Zn-0.5 wt%Ir) were studied. The segregation tendency of S in the grain boundary was investigated by Auger electron spectroscopy (AES) and Energy Dispersive Spectroscopy (EDS) with varying S concentrations. The effect of the cooling rate on the microstructures and segregation was also investigated. The compositions of several positions on the fractured surface were measured by EDS analysis. The grain refining effect of Ir was obvious when the concentration of Ir was more than 0.1%. At S-concentrations above 0.1%, Ag₂Pd₃S phase was observed at the dendrite and grain boundary due to S-segregation during solidification. This phase did not appear at 0.01% S, but the S-concentration at the grain boundary was much higher than the bulk concentration, showing S-segregation to the grain boundary. The degree of S-segregation to the grain boundary decreased slightly with grain refining. The segregation of In and Zn in A-2 alloy containing high S concentration did not occur while it occurred in the alloy with low S concentration.

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During T6 treatment of Mg-(6, 9, 12) wt.%Al alloys, X-ray diffraction analysis and microstructural observation, immersion test was performed in order to investigate the microstructural transformation and the corrosion behavior. At 220℃, the Fournelle & Clark model operates as a dominant nucleation mechanism and the nucleation rate and boundary migration velocity depends upon the solute concentration. Also the single seam nodule is observed as a dominant morphology of transformed phase. At initial precipitation stage, the corrosion rate of heat-treated specimens depends upon the galvanic corrosion between α-Mg matrix and transformed phase, as well as the corrosion behavior of α-Mg matrix itself. The corrosion behavior of α-Mg matrix depends mainly upon the repassivation tendency of exposing surface, which depends upon the Al concentration of α-Mg matrix. Although the corrosion property of solutionized alloy depends upon the corrosion behavior of α-Mg matrix, the corrosion rate of aged alloy is remarkably decreased by the formation of network morphology of transformed phase along grain boundary.

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